Because everyone keeps letting it happen. It's not illegal (yet) and the actors go for it. Acting is like gambling... Lots of people dream big, but only a few actually make it. Most of the dreamers are willing to risk everything for that chance. They'll sign any contract that gives them a chance at making it, and they think not signing a bad contract will mean they'll never work. (And that's probably true.)

The music industry works the same way, in regards to the actual content producers.